The present invention relates to a device for the fractionation of pulp, and quite particularly to a device which has a chamber, preferably of a circular horizontal cross section and having in its upper section an inlet for the pulp to be fractionated and in its lower section an outlet for the first fraction. In addition, the chamber has a screen rotatable substantially about its vertical axis, having a substantially circular cross section and being at its ends closed in relation to the chamber. On the circumference of the screen there are numerous apertures and inside it an outlet for the second fraction.
So-called pressurized screens of the above type have previously been used for the fractionation of various solids in an aqueous suspension, and in particular for the separation of cellulose fibers from coarser fiber clusters. In these known pressurized screens, solids are graded according to the particle size. They have, fitted inside a cylindrical chamber, an also cylindrical screen basket rotating about its vertical axis, with round or oblong apertures on its circumference. In these pressurized screens, efforts have been made to affect the grading result and to keep the screen apertures clean by the selection of the size, shape and frequency of the apertures in the screen basket. Efforts have been made to avoid clogging of the apertures, for example, by means of blades moving along the screen basket or by making the cylindrical surface of the screen basket uneven in shape, for example, wave-like, in order to ensure its keeping clean.
In all prior-art systems the grading is based on affecting the grading result by regulating the size, shape or frequency of the apertures in the cylindrical wall of the screen basket, so that solid particles of different sizes or shapes, such as fibers and fiber clusters, are separated from each other into different fractions. These pressurized screens known per se have a disadvantage in the tendency of the holes or slits of the screen basket to become clogged.